Oral sodium phosphate has been demonstrated in numerous clinical trials to be an effective and well-tolerated colonic cleansing agent. However, there exists a potential to induce shifts in intravascular volume. The phosphate load often results in hyperphosphatemia, which may precipitate hypocalcemia. A review at the authors' institution identified four patients with adverse events related to oral sodium phosphate. Three of these cases had pre-existing comorbidities that predisposed them to the adverse event, or had received doses higher than that used or recommended in previous trials. Recommendations for relative and absolute contraindications to the use of oral sodium phosphate are described.